In David Michaelis’s 2007 biography of Charles Schulz, the author detailed an affair that the Peanuts-comic-strip creator had in the early 70s with a young office worker while still married to his wife of nearly 20 years. Schulz was reportedly so smitten by his junior love interest, Tracey Claudius—who was 25 when she met Schulz, then 48—that he sent her notes and gifts honoring each month of their anniversary. Today, it was announced that Sotheby’s had acquired 44 love letters, including more than 20 drawings, from Claudius’s family, and will be auctioning them off on December 14.

Perhaps the most interesting detail in the A.P.’s announcement of the upcoming sale, though, is how Schulz later alluded to his relationship with Claudius in his comic strips.

In two letters from 1970 Schulz writes that he must cease calling Claudius because his long-distance phone calls to her had been discovered by his wife. Soon after, he created a strip in which Charlie Brown berated Snoopy for his obnoxious behavior when he’s not allowed to go out “to see that girl beagle.” In subsequent panels, Charlie warns Snoopy “you’d better start behaving yourself” and when Snoopy picks up the telephone, Charlie Brown yells “And stop making those long-distance phone calls.”

Schulz reportedly proposed to and was rebuffed twice by Claudius. After divorcing his first wife in 1972, the Peanutscreator wed Jean Forsyth Clyde the following year. They remained married until his death, in 2000.